The quaternary structure of M. luteus polynucleotide phosphorylase has been determined. The enzyme is an oligomer composed of three identical subunits. The enzyme undergoes conformational changes upon binding of oligo or polynucleotides which are responsible for the electrophoretic heterogeneity. The interaction of cAMP phosphodiesterase with its Ca 2 ion-dependent activator protein has been utilized to purify the activator-dependent form of the enzyme and an activator binding protein which inhibits the activation of the enzyme. BIBLIOGRAPHIC REFERENCES: Klee, C.B.: Conformational transition accompanying the binding of Ca 2 ion to the protein activator of 3'5'-cyclic adenosine monophosphate phosphodiesterase. Biochemistry 16: 1017-1024, 1977. Klee, C.B., LaJohn, L.E., Kirk, K.L., and Cohen, L.A.: 2-fluorourocanic acid, a potent reversible inhibitor of urocanase. Biochem. Biophys. Res. Comm. 75: 674-681, 1977.